Change defaults with Local<> to be more ergonomic

[newclarityex]

What problem does this solve or what need does it fill?

I often use Local for systems that keep track of local ui input, timers, counters, etc. For data that only needs to be accessed in a single system, it's very convenient especially for prototyping to not have to manage resources or components. Using Local<> with a default value however is a lot more complex and makes Local<> much less appealing to use. The workaround described in the docs is to use a resource and convert your system into a capturing closure, which is more complex.

What solution would you like?

A possible solution would be a function on Local<> to set an intial value that is only in the first run of the system.

Example:

fn counter(mut count: Local<i32>) {
    count.set_default(10);
    *count += 1;
}

What alternative(s) have you considered?

An alternative workaround is to create a wrapping type, and impl default on that type, then Local. This is still simpler than the closure solution, and makes more sense, but I feel this can be simplified further.

Example:

struct CounterLocal(i32);

impl Default for CounterLocal {
    fn default() -> Self {
        CounterLocal(10)
    }
}

fn counter(mut count: Local<CounterLocal>) {
    count.0 += 1;
}

Additional context

[alice-i-cecile]

Can you use Option as your Local type?

[newclarityex]

Using Option seems like a decent alternative as well. Trying to read + mutate seems janky but there might be a better way than what I'm doing here.

fn counter(mut count: Local<Option<i32>>) {
    let cur_count = count.unwrap_or(10);
    *count = Some(cur_count);

    // do stuff with cur_count

    *count = Some(cur_count + 1);
}

[newclarityex]

An additional edgecase are things that don't impl Default. They currently have to be wrapped to be used at all in Local.

[ilyvion]

There's Option::get_or_insert which gives you a &mut T with the given default if None; so for your example, you'd do

fn counter(mut count: Local<Option<i32>>) {
    let cur_count = count.get_or_insert(10);

    // do stuff with cur_count

    *cur_count = *cur_count + 1;
}